This invention concerns in general offshore oil and gas production from production wells in the sea bottom, and more especially new arrangements for a so-called buoy loading system. Such systems can be used instead of floating or semi-submersible production platforms, and comprise a submerged or subsurface buoy which forms a connection point for a bundle of flexible risers from production wells on the sea bottom. The buoy is designed to stand in an equilibrium position in the water body and to be able to rise and be made fast or attached to a turret system in a complementary loading vesel to establish a transport conveyor system for the petroleum products from the production wells to the loading tanks onboard the vessel.
Several designs for such buoy loading systems are known. Usually the buoy is anchored to the bottom of the sea with a plurality of anchoring chains such that the buoy is positioned in a stable equilibrium position at the desired water depth and along a vertical axis. The buoy is dimensioned such that it has sufficient buoyancy to carry the weight and the loading from anchor chains as well as the weight of the risers while assuming a predetermined neutral position, which is called a stowage position, in the water. The buoy will be given sufficient buoyancy such that it can be raised into contact with the vessel positioned above the buoy with the help of winches and wire systems, or it can be brought up under its own buoyant force. A ship fitted with a loading system as described is normally provided with a downwardly opening tunnel or shaft, which in turn is provided with a rotatable turret or rotatable system for receiving the buoy and for attaching the buoy to this turret and for attachment of the risers to piping systems in the vessel. In known constructions the anchor chains and risers are hung or attached to the bottom of the buoy. Between the buoy and the tunnel or the rotatable system in the hull structure, sealing systems can also be provided such that the buoy can be attached to the hull of the vessel providing a watertight room inside. When the buoy is attached in its seat or turret the sea water from the buoy's top side in this tunnel may be pumped out such that a dry working space on the top of the buoy can be provided. Thereby it will be easier to attach couplings between the risers in the buoy and the piping systems on the ship for transmitting petroleum fluids to the vessel's cargo tanks.
It is necessary that the vessel can rotate freely with respect to the buoy such that the vessel can turn with wind and weather about the buoy, which cannot turn because of the risers and the anchoring system. A buoy loading system can, however, to a great degree and without problems, be subjected to vertical movements and present certain advantages compared to floating and semi-submersible platforms because a loading vessel with a rotatable buoy leading system can turn according to the wind and weather to provide a robust sea-going unit. This means that production and loading can be maintained under hard weather, eventually also in areas with ice problems.
When not in use the loading buoy is stowed at a depth of water which is below all seagoing traffic, normally by about 30 meters under the surface of the sea. When it is to be used the buoy is brought up and into the rotatable turret such that the vessel can freely turn about the buoy according to the wind and weather conditions. In position in the turret, the buoy's anchoring system will also provide sufficient forces to maintain the position of the vessel under any weather conditions.
A system which makes it possible for the vessel to turn around the buoy is called a turret. The buoy is mechanically linked to the turret which can turn or roll or glide on systems on the inside of the vessel's tunnel construction and is a part of or is connected to the ship's structure.
Turrets have been used for a long time for anchoring of vessels such as drill vessels. The classically designed turret anchor system is, however, costly, complicated and difficult to maintain and to operate.
In addition, methods to connect the risers to a turreted anchored vessel are complicated, especially with a plurality of risers or a riser bundle. There exists no good solution which makes it possible to safely and rapidly disengage or connect the riser systems. According to known and published technology, subsurface buoys are connected to a vessel in a moonpool. A moonpool is a room or space over a dam formed in the open well of the vessel or recess which requires that this dam be watertight and the water above the buoy must be removed to provide a safe and dry working space. Furthermore, known designs and technology have to date not shown any means which makes it possible to adjust anchor cables or chains when the buoy is installed and attached to the vessel, and this technology also does not show any direct ways to gain access to risers or for installing these under normal operating conditions.